1. Field of the Invention
The present invention relates to a light-emitting device using electroluminescence (EL).
2. Description of the Related Art
Semiconductor lasers have been used as a light source for optical communications systems. Semiconductor lasers excel in wavelength selectivity and can emit light with a single mode. However, it is difficult to fabricate the semiconductor lasers because many stages of crystal growth are required. Moreover, types of light-emitting materials used for semiconductor lasers are limited. Therefore, semiconductor lasers cannot emit light with various wavelengths.
Conventional EL light-emitting devices which emit light with a broad spectral width have been used in some application such as for displays. However, EL light-emitting devices are unsuitable for optical communications and the like, in which light with a narrow spectral width is required.
An object of the present invention is to provide a light-emitting device which can emit light with a remarkably narrow spectral width in comparison with conventional EL light-emitting devices, and can be applied not only to displays but also to optical communications and the like.
Light-emitting Device of First Embodiment
A light-emitting device according to a first embodiment of the present invention comprises a substrate and a light-emitting device section,
wherein the light-emitting device section includes:
a light-emitting layer capable of emitting light by electroluminescence;
a pair of electrode layers for applying an electric field to the light-emitting layer;
an optical section for propagating light emitted in the light-emitting layer in a specific direction; and
an insulation layer disposed between the pair of electrode layers, having an opening formed in part of the insulation layer and capable of functioning as a current concentrating layer for specifying a region through which current supplied to the light-emitting layer flows through a layer in the opening,
wherein the optical section forms photonic bandgaps capable of inhibiting three dimensional spontaneous emission of light and includes a defect section which is set so that an energy level caused by a defect is within a specific emission spectrum, and
wherein the light emitted in the light-emitting layer is emitted with spontaneous emission being inhibited in three dimensions by the photonic bandgaps.
According to this light-emitting device, electrons and holes are injected into the light-emitting layer respectively from the pair of electrode layers (cathode and anode). Light is emitted when the molecules return to the ground state from the excited state by allowing the electrons and holes to recombine in the light-emitting layer. At this time, light with a wavelength in the photonic bandgaps cannot be propagated through the optical section. Only light with a wavelength equivalent to the energy level caused by the defect is propagated through the optical section. Therefore, light with a narrow emission spectrum width with an inhibited three-dimensional spontaneous emission can be obtained with high efficiency by specifying the width of the energy level caused by the defect.
According to the first light-emitting device, since the insulation layer functions as a current concentrating layer in the light-emitting device section, the region through which current supplied to the light-emitting layer flows can be specified. Therefore, current intensity and current distribution can be controlled in the region from which light is to emit, whereby light can be emitted with high emission efficiency.
In the case where the insulation layer functions as cladding, assuming that the waveguide formed of a light-emitting layer as a core and an insulation layer as cladding, the guide mode of light propagated toward the waveguide section through the optical section can be controlled by specifying the opening of the insulation layer. Specifically, the guide mode of light propagated through the light-emitting layer (core) can be set at a specific value by specifying the width of the region in which light is confined (width perpendicular to the direction in which light is propagated) by the insulation layer (cladding) The relation between the guide mode and the waveguide is generally represented by the following equation.
Nmax+1xe2x89xa7K0xc2x7axc2x7(n12xe2x88x92n22)1/2/(xcfx80/2) 
where:
K0:2xcfx80/xcex,
a: half width of waveguide core,
n1: refractive index of waveguide core,
n2: refractive index of waveguide cladding, and
Nmax: maximum value of possible guide mode.
Therefore, if the parameters of the above equation such as the refractive indices of the core and cladding have been specified, the width of the light-emitting layer (core) specified by the width of the opening of the current concentrating layer may be selected depending on the desired guide mode. Specifically, the width (2a) of the light-emitting layer corresponding to the core in a desired guide mode can be calculated from the above equation by substituting the refractive indices of the light-emitting layer provided inside the current concentrating layer and the insulation layer (current concentrating layer) for the refractive indices of the core and cladding of the waveguide, respectively. The suitable width of the core layer of the waveguide section to which light is supplied from the light-emitting device section can be determined taking into consideration the resulting width of the light-emitting layer, calculated value obtained from the above equation based on the desired guide mode, and the like. Light with a desired mode can be propagated from the light-emitting device section toward the waveguide section with high combination efficiency by appropriately specifying the width of the light-emitting layer, width of the core layer, and the like. In the light-emitting device section, light-emitting layer in the current concentrating layer formed of the insulation layer may not uniformly emit light. Therefore, the specific values for each member such as the light-emitting layer, optical section, and waveguide section can be suitably adjusted based on the width (2a) of the core (light-emitting layer), determined using the above equation, so that each member exhibits high combination efficiency.
The guide mode of the light-emitting device can be set to 0 to 1000. In particular, when used for communications, the guide mode can be set to about 0 to 10. Light with a specific guide mode can be efficiently obtained by specifying the guide mode of light in the light-emitting layer in this manner.
As described above, according to the present invention, a light-emitting device which substantially has three-dimensional photonic bandgaps structure can emit light with a remarkably narrow spectral width in comparison with conventional EL light-emitting devices and exhibiting directivity, and can be applied not only to displays but also optical communications and the like, can be provided.
Light-emitting Device of Second Embodiment
A light-emitting device according to a second embodiment of the present invention comprises comprising a light-emitting device section and a waveguide section propagating light from the light-emitting device section which are integrally formed on a substrate,
wherein the light-emitting device section includes:
a light-emitting layer capable of emitting light by electroluminescence;
a pair of electrode layers for applying an electric field to the light-emitting layer;
an optical section for propagating light emitted in the light-emitting layer in a specific direction; and
an insulation layer disposed between the pair of electrode layers and capable of functioning as a cladding layer,
wherein the waveguide section includes:
a core layer continuously formed with part of the optical section; and
a cladding layer continuously formed with the insulation layer, and
wherein the optical section forms photonic bandgaps capable of inhibiting three dimensional spontaneous emission of light and includes a defect section which is set so that an energy level caused by a defect is within a specific emission spectrum, and
wherein the light emitted in the light-emitting layer is emitted with spontaneous emission being inhibited in three dimensions by the photonic bandgaps.
According to the second light-emitting device, at least part of the optical section in the light-emitting device section and the core layer in the waveguide section are integrally formed, and the insulation layer (cladding layer) in the light-emitting device section and the cladding layer in the waveguide section are integrally formed. Therefore, the light-emitting device section and the waveguide section are optically connected with high combination efficiency, thereby ensuring efficient light propagation.
In this configuration, as the material for the insulation layer, a material which functions as a cladding layer for light with a specific wavelength is selected. According to this light-emitting device, since at least part of the optical section in the light-emitting device section and the core layer in the waveguide section can be formed and patterned in the same step, fabrication can be simplified. The insulation layer (cladding layer) in the light-emitting device section and the cladding layer in the waveguide section can be formed and patterned in the same step. This also simplifies the fabrication.
According to the present invention, a light-emitting device, which substantially has a three-dimensional photonic band gap structure, can emit light having a remarkably narrower spectral width than conventional EL light-emitting devices and exhibiting directivity, and can be applied not only to displays but also to optical communications and the like can be provided in the same manner as in the first light-emitting device.
In the light-emitting device of the first or second embodiment, the opening formed in the insulation layer and functioning as a current concentrating layer or a cladding layer may be formed so as to face the optical section. The opening may be a slit extending in the periodic direction of the first optical section, specifically, in the direction in which light is waveguided. It is appropriate that at least part of the light-emitting layer be present in the opening formed in the insulation layer. According to this configuration, the region of the light-emitting layer to which current is to supplied and the region specified by the current concentrating layer can be self-alignably positioned.
The light-emitting device according to the present invention may have the following structure.
In the light-emitting device, the optical section may comprise:
a first optical section having a periodic refractive index distribution in at least two directions on XY surface and capable of forming two-dimensional photonic bandgaps; and
a second optical section having a periodic refractive index distribution in at least Z direction and capable of forming at least one-dimensional photonic bandgaps, and
the defect section may be formed in the first optical section, and light may be emitted in one direction on the XY surface of the first optical section.
According to this light-emitting device, light with a very narrow emission spectrum width with a three-dimensional spontaneous emission being inhibited can be obtained with high efficiency by the combination of the first optical section which inhibits a two-dimensional light propagation at the X-Y surface and the second optical section which inhibits a one-dimensional light propagation at least in the Z direction.
The second optical section may have a structure such as a grating-shaped structure, a multilayer film structure, a columnar or mosaic columnar-shaped structure, or a combination of these structures.
Specifically, the second optical section includes first medium layers and second medium layers alternately arranged. Therefore, the second optical section has a periodic refractive index distribution in the Z direction, thereby forming one-dimensional photonic bandgaps. The second optical section may have a periodic refractive index distribution in each of the x, Y, and Z directions in which the first medium layers and the second medium layers are alternately arranged, thereby forming a three-dimensional photonic band. The second optical section may include a plurality of unit cells of the diamond structure and form three-dimensional photonic bandgaps.
Specifically, the first optical section may include columnar-shaped first medium layers arranged in the shape of tetragonal lattice and second medium layers formed between the first medium layers, and have a periodic refractive index distribution in the first and second directions. Photonic bandgaps with inhibited spontaneous emission in the two directions at the XY surface can be formed by this first optical section.
The first optical section may include columnar-shaped first medium layers arranged in the shape of a triangle lattice or a honeycomb lattice, for example, and second medium layers formed between the first medium layers, and have a periodic refractive index distribution in the first, second, and third directions at the XY surface. Photonic bandgaps with inhibited spontaneous emission in the three directions at the X-Y surface can be formed by this first optical section.
The light-emitting layer may include an organic light-emitting material as the light-emitting material. Use of the organic light-emitting material widens selection of materials and enables emission of light with various wavelengths in comparison with the case of using a semiconductor material or inorganic material, for example.
Some of the materials which can be used for each section of the light-emitting device according to the present invention are illustrated below. These materials are only some of the conventional materials. Materials other than these materials may also be used.
Light-Emitting Layer
The material for the light-emitting layer is selected from conventional compounds in order to obtain light with a specific wavelength. As the material for the light-emitting layer, any organic or inorganic compound may be used. Of these, organic compounds are suitable in view of availability of a wide variety of compounds and film-formability.
As examples of such organic compounds, aromatic diamine derivatives (TBD), oxydiazole derivatives (PBD), oxydiazole dimers (OXD-8), distyrylarylene derivatives (DSA), beryllium-benzoquinolinol complex (Bebq), triphenylamine derivatives (MTDATA), rubrene, quinacridone, triazole derivatives, polyphenylene, polyalkylfluorene, polyalkylthiophene, azomethine zinc complex, polyphyrin zinc complex, benzooxazole zinc complex, and phenanthroline europium complex which are disclosed in Japanese Patent Application Laid-open No. 10-153967, and the like can be given.
As the material for the organic light-emitting layer, conventional materials disclosed in Japanese Patent Application Laid-open No. 63-70257, No. 63-175860, No. 2-135361, No. 2-135359, No. 3-152184, No. 8-248276, No. 10-153967, and the like can be used. These compounds may be used either individually or in combination of two or more.
As examples of inorganic compounds, ZnS:Mn (red region), ZnS:TbOF (green region), SrS:Cu, SrS:Ag, SrS:Ce (blue region), and the like can be given.
Optical Waveguide
The optical waveguide includes a layer which functions as a core, and a layer which has a refractive index lower than that of the core and functions as cladding. Specifically, these layers include the optical section (core) and the insulation layer (cladding) in the light-emitting device section, the core layer and the cladding layer in the waveguide section, substrate (cladding), and the like. Conventional inorganic and organic materials may be used for the layers for forming the optical waveguide.
Typical examples of inorganic materials include TiO2, TiO2-SiO2 mixture, ZnO, Nb2O5, Si3N4, Ta2O5, HfO2, and ZrO2 disclosed in Japanese Patent Application Laid-open No. 5-273427.
As typical examples of organic materials, various conventional resins such as thermoplastic resins, thermosetting resins, and photocurable resins can be given. These resins are appropriately selected depending on the method of forming the layer and the like. For example, use of a resin cured by energy of at least one of heat or light enables utilization of commonly used exposure devices, baking ovens, hot plates, and the like.
As examples of such materials, a UV-curable resin disclosed in Japanese Patent Application No. 10-279439 by the applicant of the present invention can be given. As UV-curable resins, acrylic resins are suitable. UV-curable acrylic resins having excellent transparency and capable of curing in a short period of time can be obtained by using commercially-available resins and photosensitizers.
As specific examples of basic components of such UV-curable acrylic resins, prepolymers, oligomers, and monomers can be given.
Examples of prepolymers or oligomers include acrylates such as epoxy acrylates, urethane acrylates, polyester acrylates, polyether acrylates, and spiroacetal-type acrylates, methacrylates such as epoxy methacrylates, urethane methacrylates, polyester methacrylates, and polyether methacrylates, and the like.
Examples of monomers include monofunctional monomers such as 2-ethylhexyl acrylate, 2-ethylhexyl methacrylate, 2-hydroxyethyl acrylate, 2-hydroxyethyl methacrylate, N-vinyl-2-pyrrolidone, carbitol acrylate, tetrahydrofurfuryl acrylate, isobornyl acrylate, dicyclopentenyl acrylate, and 1,3-butanediol acrylate, bifunctional monomers such as 1,6-hexanediol diacrylate, 1,6-hexanediol dimethacrylate, neopentyl glycol diacrylate, neopentyl glycol dimethacrylate, ethylene glycol diacrylate, polyethylene glycol diacrylate, and pentaerythritol diacrylate, and polyfunctional monomers such as trimethylolpropane triacrylate, trimethylolpropane trimethacrylate, pentaerythritol triacrylate, and dipentaerythritol hexaacrylate.
These inorganic and organic materials are illustrated taking only light confinement into consideration. In the case where the light-emitting device section has a light-emitting layer, hole transport layer, electron transport layer, and electrode layer, and at least one of these layers functions as the core or cladding layer, the materials for these layers may be employed as the material for the layers of the optical waveguide.
Hole Transport Layer
In the case of using an organic light-emitting layer in the light-emitting device section, a hole transport layer may be formed between the electrode layer (anode) and the organic light-emitting layer, as required. As the materials for the hole transport layer, materials conventionally used as hole injection materials for photoconductive materials or materials used for a hole injection layer of organic light-emitting devices can be selectively used. As the materials for the hole transport layer, any organic or inorganic substance having a function of either hole injection or electron barrier characteristics may be used. As specific examples of such substances, substances disclosed in Japanese Patent Application Laid-open No. 8-248276 can be given.
Electron Transport Layer
In the case of using an organic light-emitting layer in the light-emitting device section, an electron transport layer may be formed between the electrode layer (cathode) and the organic light-emitting layer, as required. Materials for the electron transport layer are only required to have a function of transferring electrons injected from the cathode to the organic light-emitting layer. Such materials can be selected from conventional substances. For example, a substance disclosed in Japanese Patent Application Laid-open No. 8-248276 can be given as a specific example.
Electrode Layer
As the cathode, electron injectable metals, alloys, electrically conductive compounds with a small work function (for example, 4 eV or less), or mixtures thereof can be used. Materials disclosed in Japanese Patent Application Laid-open No. 8-248276 can be given as specific examples of such electrode substances.
As the anode, metals, alloys, electrically conductive compounds with a large work function (for example, 4 eV or more), or mixtures thereof can be used. In the case of using optically transparent materials as the anode, transparent conductive materials such as CuI, ITO, SnO2, and ZnO can be used. In a case where transparency is not necessary, metals such as gold can be used.
The optical section can be formed by conventional methods without specific limitations. Typical examples of such methods are given below.
(1) Lithographic Method
A positive or negative resist is exposed to ultraviolet rays, X-rays, or the like and developed. Then the resist layer is patterned to form an optical section. As a patterning technology using a resist formed of polymethylmethacrylate or a novolak resin, technologies disclosed in Japanese Patent Applications Laid-open No. 6-224115 and No. 7-20637 can be given.
As a technology of patterning a polyimide using photolithography, technologies disclosed in Japanese Patent Applications Laid-open No. 7-181689 and No. 1-221741, and the like can be given. Furthermore, Japanese Patent Application Laid-open No. 10-59743 discloses a technology of forming an optical section of polymethylmethacrylate or titanium oxide on a glass substrate utilizing laser ablation.
(2) Formation of Refractive Index Distribution by Irradiation
The optical waveguide section of the optical waveguide is irradiated with light having a wavelength which produces changes in the refractive index to periodically form areas having different refractive indices on the optical waveguide section, thereby forming an optical section. As such a method, it is appropriate to form an optical section by forming a layer of polymers or polymer precursors and polymerizing part of the polymer layer by irradiation or the like to periodically form areas having different refractive indices. Such a technology is disclosed in Japanese Patent Applications Laid-open No. 9-311238, No. 9-178901, No. 8-15506, No. 5-297202, No. 5-39480, No. 9-211728, No. 10-26702, No. 10-8300, and No. 2-51101, and the like.
(3) Stamping Method
An optical section is formed by, for example, hot stamping using a thermoplastic resin (Japanese Patent Application Laid-open No. 6-201907), stamping using an UV curable resin (Japanese Patent Application Laid-open No. 10-279439), or stamping using an electron-beam curable resin (Japanese Patent Application Laid-open No. 7-235075).
(4) Etching Method
A thin film is selectively patterned using lithography and etching technologies to form an optical section.
The methods for forming the optical section are described above. Specifically, the optical section formed of at least two regions, each having a different refractive index, and can be fabricated, for example, by a method of forming these two regions from two materials having different refractive indices, a method of forming the two regions from one material by partly modifying the material forming one of the two regions so that the two regions have different refractive indices, and the like.
Each layer of the light-emitting device can be formed by a conventional method. For example, each layer of the light-emitting device is formed using a suitable film-forming method depending on the materials therefor. As specific examples of such a method, a vapor deposition method, spin coating method, LB method, ink jet method, and the like can be can be given.